1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, or a facsimile, for forming images according to an image forming process using an electrophotographic system or an electrostatic recording system.
2. Description of the Related Art
Conventionally, it has been desired that an image forming apparatus such as a copying machine or printer using an electrophotographic system should reproduce images from the same image data on a transfer material at a constant density.
To this end, the image forming apparatus forms a specific pattern of developed images (patches) for density detection on an image bearing body such as a photosensitive drum at a preliminary stage prior to the image forming process, that is, during a pre-rotation period upon completion of warm-up operation after startup. The image forming apparatus then reads out the density of the formed pattern, and based on the read density values, changes various parameters for determining the image forming conditions of the image forming apparatus, such as the amount of charge, the amount of exposure, an image information conversion table or so-called lookup table (hereinafter called “LUT”), a development field, and the amount of developer supply. Thus the image forming apparatus performs image control (pre-rotational image control) to stabilize the quality of images being formed.
Even when the tone characteristics of the image forming apparatus have changed due to changes in environmental conditions, this pre-rotational image control can be performed by forming patches on the image bearing body and reading their densities again to feedback the results to the various parameters for determining the image forming conditions, thereby stabilizing image quality in response to the changes in environmental conditions.
In the above-mentioned image forming method, however, the density may vary from image to image, because the image forming apparatus may not be able to respond to characteristics constantly varying due to long hours of image formation in the course of a day's work, such as changes in dark/light potential parts of the image bearing body or photosensitive drum, and changes in the amount of charge of developer. The variations in image density are brought to the fore as variations in color especially in such a color image forming apparatus that is required to carry out high-quality image formation.
In addition, since time-consuming and troublesome work is required for the above-mentioned pre-rotational image control, it is difficult to frequently carry out this control.
As a measure to solve this problem, a method called post-rotational image control is known as described in Japanese Patent Application Laid-Open No. 2000-238341 (corresponding to U.S. Pat. No. 6,418,281). In this method, one or more predetermined tone patterns are formed as patches on an image bearing body at intervals of a predetermined number of sheets after repetitions of the image forming process, and the densities of the tone patterns are read to control various parameters according to the read values.
Another method called inter-sheet image control is known as described in Japanese Patent Application Laid-Open No. 2003-202711 (corresponding to U.S. Pat. No. 2,003,128381 and EP-1326426). In this method, one or more tone patterns are formed in the same manner mentioned above on an image bearing body between image forming cycles at the time of continuous image formation at which images are continuously formed on multiple sheets, that is, at an interval between printing sheets, and the densities of the tone patterns are read to control various parameters according to the read values.
However, since the post-rotational image control is performed after image formation on a predetermined number of sheets, no image cannot be formed during the control period, reducing user productivity.
On the other hand, since the inter-sheet image control method forms patches between printing sheets, it requires a longer time to perform the control at an interval between printing sheets, causing a reduction in the number of formed images producible at the same processing rate.
In both of the foregoing image control methods, the image patterns are formed as patches other than images desired by the user to be formed in the image forming process, causing extra consumption of developer and promotion of contamination in the image forming apparatus.